Get your finger ready—and some caulk, spray foam, or acoustical sealant
Where the wall plates of interior partition walls come up to the bottom of the ceiling joist, we’ve got gaps between the drywall and the plates themselves, and those plates are going to run either perpendicular or parallel to the ceiling joists. Where we have it perpendicular, the wall plate you can see in the drywall. And over here, where we have it parallel, they put in 2×6 blocking that supports the ceiling drywall but sits on top of that top plate.
There are slightly different approaches to sealing both of those situations. Between the drywall and the top plate of the wall, we’ve got a few different choices of materials we can use to seal up. Probably the simplest and easiest is just a really good, high-quality caulking. You want something that’s elastomeric, which means that it’s stretchy after it seals up. I tend to go for something that’s a little less expensive but works just as well, which is an acoustical sealant. It usually comes in larger tubes, and you find it at commercial suppliers. The stuff is very sticky, and it doesn’t completely cure—and that’s important. It’ll skin over, but it’ll stay sticky, stay adhesive, and be able to move with the materials as moisture, temperature, and so on change the dynamics of the material.
Now, another choice would be a high-expansion or low-expansion spray foam. One of the challenges with this stuff is that it’s only good when you have a fairly large gap, about 1/4 to 1/2 inch. It doesn’t work really well for these drywall-to-framing connections because the gaps are really small — maybe 1/8 inch, 1/16 inch. That’s why I…